1. Field of the Invention.
The present invention relates to the field of gas plasma treating the plastic materials and, more particularly, to gas plasma etching of drilled, multilayer printed wiring boards prior to metal plating of conductive layer interconnections.
2. Description of the Prior Art.
Multilayered printed circuit or wiring boards, whether rigid or flexible, are typically constructed having a number of layers of copper conductor patterns separated by layers of insulating plastic. Between the plastic and copper layers, an organic adhesive layer, e.g. epoxy, phenolic butyral, acrylic etc., is used to form an adhesive bond. When fabrication of the boards is complete, numbers of small holes or apertures are drilled through the boards in particular locations at which two or more copper layers are to be interconnected. A metallic conductor, for example copper, is then plated into the apertures to make electrical contact between exposed edge portions of the copper conductive layers to thereby interconnect such layers.
During the drilling operation, however, the drill frequently smears a thin layer or coating of plastic and/or adhesive around the inside of the apertures. These smeared layers, which may cover all or most of the otherwise exposed edges of the copper layers, tend to insulate such edges and, if not completely removed prior to the interconnect metal plating, prevent good electrical contact from being made with the copper layers. Hence, good electrical interconnecting between the copper layers is prevented. In addition, small fragments of plastic, torn from the circuit board during the drilling operation, may project into the apertures and cause similar effects.
Complete removal of these smeared, insulating layers and torn plastic fragments is thus essential prior to plating of the interconnecting metal. Heretofore, such smeared layers and particle removal has been accomplished by chemically etching the drilled multilayered boards with strong liquid chemicals, for example, concentrated sulphuric acid, strong chromic acid and/or strong alkaline solutions, which dissolve the smeared material (as well as small amounts of the non-metallic laminations wherever exposed). Such wet etching procedure has been required to be comparatively lengthy and complex to assure complete removal from the apertures of the smeared material and particles, and to assure removal of all traces of the etching chemicals used in order to prevent subsequent damage to the boards and electronic components mounted thereto. A typical pre-etching, etching and cleaning or neutralizing process thus comprises as many as 10 to 30 separate steps, for example, including various water rinses, chromate neutralizing and other rinsing steps, including an acid dip, and is accordingly very timeconsuming and expensive. Also wet chemical etching still often results in poor metal plating adhesion and thus high board rejection rates. In addition, certain materials, such as Dupont's Pyralux, used in the construction of flexible printed circuit boards are not compatible with the commonly used wet chemical etchants. Obviously, there is much need for improved, less complicated and less time-consuming processes for removing the smeared layers from drilled apertures in printed circuit boards and the like.
Within the past few years, many wet chemical etching processes have been replaced to advantage by dry gas plasma etching processes. Some of the gas molecules in the plasma are ionized; many others are ruptured to create a large number of free radicals that is, molecular fragments containing unpaired electrons which are not involved in chemical bonding. This presence of ions and particularly the free radicals makes the plasma extremely reactive (thus effective as an etchant). The plasma state is, by its nature, so unstable that energy must be continuously supplied to maintain it. Typical applications of such plasma etching include surface etching of comparatively stable plastics, such as Teflon, having surfaces which will not otherwise accept paints, dyes, inks, adhesives etc. Previously such surface treatment was by chemical etching or mechanical abrasion.
The most useful plasmas for such etching operations have been found to be low temperature, low pressure plasmas--plasmas having temperatures typically in a range from about 50.degree.-200.degree. or 250.degree. C. and pressure in the range of from about 0.1-200 torr, and in which usually fewer than about 1% of the gas particles may be ionized. Such may contain more than 90% free radicals. Because such plasmas neither require nor generate excessive heat during operation, they can safely be used to treat materials, such as plastics, without adversely affecting their structural properties. The equipment employed for generating and using low temperature, low pressure plasmas is relatively simple and avoids major problems associated with the wet chemical etching processes; since the plasma reaction products are gaseous in form, substantially no solid residues are left upon completion of the plasma etching process. Typically the gases used in such plasma etching processes are oxygen (O.sub.2) or a mixture of oxygen and carbon tetrafluoride (CF.sub.4), the carbon tetrafluoride serving as a source of atomic fluorine, a powerful oxidizing agent.
Some use has accordingly been made of plasma etching techniques in the electronics industry. For example, the Jacob U.S. Pat. No. 3,806,365 discloses use of a plasma of oxygen and organohalides to remove photoresist and its associated inorganic contaminants from surfaces of semiconductor materials. LaCombe U.S. Pat. Nos. 3,816,196 and 3,816,198 disclose use of an oxygen plasma to etch patterns onto plastic films through a photoresist mask, for microcircuit fabrication. The Irving U.S. Pat. No. 3,615,956 discloses the use of an oxygen and carbon tetrafluoride plasma to polish and clean silicon wafers, to etch break lines in the wafers and to locate pin holes in insulating oxide films on the wafer surfaces. An article by Bersin entitled "The Fourth State", P. 60, Industrial Research, April 1975 generally described the properties and uses of plasma in the treating of surfaces.
However, insofar as is known, no disclosure has heretofore been made of utilizing a gas plasma for treating multilayer printed wiring or circuit boards (of either the rigid or flexible type) to remove smeared layers and torn plastic fragments in drilled holes or to etch-back the non-metallic layers prior to metal plating of conductive layer interconnects.